md: factor out a helper to choose sync action from md_check_recovery()

There are no functional changes, on the one hand make the code cleaner,
on the other hand prevent following checkpatch error in the next patch to
delay choosing sync action to md_start_sync().

ERROR: do not use assignment in if condition
+       } else if ((spares = remove_and_add_spares(mddev, NULL))) {

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Xiao Ni <xni@redhat.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20230825031622.1530464-3-yukuai1@huaweicloud.com